A novel GRAS transcription factor, ZmGRAS20, regulates starch biosynthesis in rice endosperm

Starch occupies the maximal component of cereal grains and is pivotal for maize yield and quality. However, the regulatory mechanism of starch synthesis is still poorly understand. In this study, a GRAS transcription factor, ZmGRAS20, was isolated from maize inbred line B73 based on transcriptome sequencing. Quantitative real-time PCR indicated that ZmGRAS20 is specifically expressed in maize endosperm. Transient expression of ZmGRAS20-green fluorescent protein fusion protein in tobacco cells showed a nucleus and membrane localization of the protein. Transactivation assay of ZmGRAS20 demonstrated that it has no transactivation activity in yeast cells. Overexpression of ZmGRAS20 led to a chalky region of ventral endosperm with decreased starch content and defective agronomic characters in transgenic seeds. Moreover, ZmGRAS20-overexpression plants had fewer fractions of long-branched starch chains. Further scanning electron microscopy observation of ZmGRAS20 transgenic seeds exhibited altered starch granules morphology compared with wide type plants. Taken together, these results suggested that ZmGRAS20 may function as a starch synthesis regulatory factor in rice endosperm.